Advertisement

Advertisement

Underwater neutrino telescope looks down to see sky

Construction of the ANTARES telescope, which will search for neutrinos under the Mediterranean Sea, is now complete

(Image: ANTARES)

An international team has finished building the ANTARES telescope, which will search for elusive particles called neutrinos from its base 2.5 kilometres under the Mediterranean Sea.

The telescope will run for roughly five years. If all goes well, ANTARES’s design might be used to build a larger version to rival the cubic-kilometre-sized IceCube Neutrino Observatory currently under construction in the ice at the South Pole. The giant projects would complement one another, since they would each look through the Earth at different parts of the sky.

Neutrinos are fundamental particles that are produced in many natural particle accelerators, such as supernovae and the ultra-bright centres of galaxies. Physicists also expect to see neutrinos produced by the annihilation of dark matter, a process that may occur within the Sun.

Advertisement

But neutrinos interact extremely weakly with matter, so they rarely collide with atoms and are therefore difficult to observe. So far, detectors have only been able to conclusively find neutrinos from our own Sun and from one nearby supernova, 1987A, says particle astrophysicist John Beacom of Ohio State University in Columbus, US.

Finding so-called cosmic neutrinos could tell us something about the internal dynamics of the objects that created them, he says&colon; “We want to know how the engine works in detail. That’s why it’s crucial to measure this emission.”

To find evidence of neutrinos, physicists must wait for the particles to knock into other matter. The collision produces a charged particle called a muon, which produces a flash of detectable light when it passes through something transparent, like ice or water.

Buried deep

The trick is to find ice or water that is relatively deep. That’s because charged particles from space called cosmic rays produce neutrinos and muons when they hit the atmosphere.

Almost all of the neutrinos produced in the Earth’s atmosphere travel through the planet unhindered. This makes it tricky to distinguish atmospheric neutrinos from cosmic neutrinos.

But the relatively low-energy muons produced in the atmosphere can travel only a few kilometres. So by building neutrino arrays like ANTARES and IceCube deep under water or ice, astronomers can make sure that most of the muons they detect were actually produced by cosmic neutrinos.

What’s more, since these detectors can look down through the Earth to see the universe, using the whole planet as a shield to absorb the riffraff of particles from the atmosphere, telescopes in different hemispheres can better observe neutrinos coming from all over the sky.

Galactic centre

From its home near the bottom of the Mediterranean off the French coast near Toulon, ANTARES observes the southern sky. That includes the centre of the Milky Way, a crowded region expected to boast a number of cosmic neutrino sources.

ANTARES uses 12 lines of vertically strung detectors that together span roughly 200 by 200 metres. Each line is weighted down by 1.5 tonnes of iron, and if ANTARES needs servicing, the team can acoustically trigger release hooks to pop the telescope from its anchor, allowing it to float to the surface.

The last line of detectors was laid down in May, and so far, ANTARES has spotted hundreds of neutrinos. That number seems consistent with what would be created in the atmosphere, says ANTARES spokesperson John Carr of the Centre for Particle Physics in Marseilles, France.

‘Stepping stone’

But as the team begins to pinpoint where these particles came from, they might find that some are cosmic neutrinos. “If we see a few neutrinos a year from extraterrestrial sources, we’d be very excited,” Carr told New Scientist. “That would be a significant discovery.”

In addition to ANTARES, two other neutrino telescopes are being built in the Mediterranean. All three are pilot projects for a northern hemisphere neutrino telescope that, like IceCube at the South Pole, will also be a cubic kilometre in size. Other astronomers are setting their sights on Lake Baikal, where plans to build a telescope of similar size are in the works.

Because the projects would cover such a large volume of space, they offer a better chance of catching rare cosmic neutrinos than their smaller counterparts.

ANTARES is “interesting in its own right, but it’s a stepping stone to something even bigger”, Beacom told New Scientist. “We have very promising indications that a kilometre-size cube is where things really start to get interesting.”